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Subject: Re: SPICE and amp stability
NNTP-Posting-Date: Thu, 21 Nov 2002 17:16:57 GMT
Organization: AT&T Broadband
Date: Thu, 21 Nov 2002 17:16:57 GMT
Jim Thompson, analog wrote:
>> You obviously are a Pspice aficionado, Jim. There is a rather clever
>> way to force any circuit containing behavioral models, no matter how
>> non-linear, to always successfully reach an initial operating point
>> solution. It takes full advantage of Pspice's scheme of cutting back
>> all power source as a convergence seeking ploy together with a unity
>> voltage source as a reference node for taming the behavioral models.
>> Can you guess what it is?
> You lost me there "analog", what are you driving at? Parameterizing
> behavioral gains proportionate to power supplies??
Almost. Have you ever modeled the low frequency behavior of switching
power supplies using average duty cycle controlled sources to replace
the semiconductor switches? One essentially use E and G sources to
make a "dc transformer" with a dynamic turns ratio that depends on the
node voltage representing duty cycle. These behavioral expressions
contain product terms, but what really gives the simulator fits are
higher order expressions with both numerator and denominator terms and
with terms that include sums, differences, especially involving
constants. There are lots of non-linear real world devices that don't
have a suitable built-in model in Pspice the behavior of which can,
nevertheless, be captured quite well via behavioral modeling. Tunnel
diodes, vacuum tube, arc discharges, high level control system blocks
all come to mind.
However, there are many ways to express essential the same model within
Pspice. The trick is to come up with one that doesn't give the
simulator fits when either it is searching for an initial solution to
the system matrix or when it is running a transient simulation. That
is what my question was about. -- analog